1. Field of the Invention
The present invention relates, in general, to an end plug for a dual-cooled fuel rod and, more particularly, to an intermediate end plug assembly for a segmented fuel rod, capable of stably supporting the fuel rod to the end of its cycle even if an interval between the fuel rods becomes narrow due to application of a dual-cooled fuel rod, and reducing excess vibration induced by flows of interior and exterior channels of the dual-cooled fuel rod for obtaining high burnup and output.
2. Description of the Related Art
A nuclear fuel assembly is charged in the core of a pressurized water reactor. This nuclear fuel assembly is composed of a plurality of fuel rods, in each of which a cylindrical uranium sintered compact (or a cylindrical uranium pellet) is inserted.
The fuel rods can be divided into two types, cylindrical and annular, according to shape. The annular fuel rods are called dual-cooled fuel rods.
In comparison with the pellet of the cylindrical fuel rod, the pellet of the annular fuel rod has a low internal temperature due to a thinner thickness and a wider heat transfer area, and thus a relatively higher safety margin.
FIG. 1 is a schematic front view illustrating a conventional cylindrical nuclear fuel assembly. Referring to FIG. 1, the nuclear fuel assembly 10 includes fuel rods 11, spacer grids 15, guide thimbles 13, an upper end fitting 17 and a lower end fitting 16.
Each fuel rod 11 has a structure in which a uranium sintered compact or a uranium pellet (not shown) generating high-temperature heat through nuclear fission is enclosed by a zirconium alloy cladding tube.
Each fuel rod 11 has upper and lower end plugs 18 and 19 coupled to lower and upper portions thereof so as to prevent inert gas filled between the cladding tubes thereof from leaking out.
Meanwhile, the fuel rod 11 is a structure having a considerably long length compared to the diameter thereof. When this structure having a great elongation ratio is put under the flow of a coolant, the fuel rod 11 causes flow-induced vibrations due to the flow of the coolant. Thus, in order to reduce this flow-induced vibration, the structure called a spacer grid 15 is installed in a predetermined section with respect to the entire length of the fuel rods 11 so as to support the fuel rods 11, thereby preventing the fuel rods 11 from being vibrated by the flow of the coolant.
However, in the case of the dual-cooled fuel rod designed to charge nuclear fuel into an annular space defined by a dual tube of inner and outer tubes, the spacer grid taking charge of an important function of inhibiting the vibration of the fuel rods caused by the flow of the coolant has no choice but to support only the outer tube of each fuel rod due to its structure. Due to the limitation of this supporting structure, in the case of the inner tube having the elongation ratio of about 400 or more, only opposite ends of each fuel rod are supported by the upper and lower end plugs.
Of course, in the case of the dual-cooled fuel rod, a uranium dioxide (UO2) pellet exists between the inner and outer tubes. Thus, the vibration of the inner tube is expected to be inhibited to a certain extent. However, in considering the fuel rod having the elongation ratio of about 400 or more, it is easily expected that a vibration amplitude of the inner tube is remarkably great, as compared to the outer tube having numerous support points formed in an axial direction of the fuel rod by the spacer grid.
Further, in the case of the typical fuel rod as shown in FIG. 2, since a coolant channel is formed outside the fuel rod, a phenomenon in which the flow of the coolant in the reactor core is restricted mainly occurs due to foreign materials in the reactor core. Thus, since cooling performance of the fuel rod is sufficiently maintained if the foreign materials are screened to a certain extent before they enter the reactor core, such a phenomenon is mainly overcome by additionally installing an apparatus for filtering the foreign materials on the lower end fitting or a support for filtering the foreign materials above the lower end fitting.
However, in the case of the dual-cooled fuel rod which obtains economical effects by lowering a central temperature of the nuclear fuel to secure stability of the nuclear fuel at a very high burnup together with high output, another problem occurs. This is because, in the case of the dual-cooled fuel rod, the coolant channel is formed outside the fuel rod, but the coolant flows in the fuel rod in order to increase the cooling performance, so that the coolant channel is formed inside the fuel rod. This coolant channel formed inside the fuel rod has an advantage in that it increases the cooling performance and thus the output of the nuclear fuel, but it has a disadvantage in that, if it is blocked, the fuel rod is greatly exposed to a danger of departure from nuclear boiling ratio (DNBR). Particularly, since the internal coolant channel of the dual-cooled fuel rod has a very narrow flow cross-sectional area, it is in high danger of being blocked although only small foreign materials get into it.
In this manner, if the internal coolant channel of the dual-cooled fuel rod is blocked and as a result a smooth flow of the coolant is obstructed, the coolant does not flow in the fuel rod, and thus is stagnant. Thereby, the dual-cooled fuel rod is exposed to the danger of DNBR. However, since the internal coolant channel does not exist in the conventional fuel rod, a new resolution to the dual-cooled fuel rod must be found.